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doc: initial PDP10 ELF relocation documentation

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doc/pdp10-relocation.txt
Copyright (C) 2020 Mikael Pettersson
This file is part of pdp10-tools.
pdp10-tools is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
pdp10-tools is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with pdp10-tools. If not, see <http://www.gnu.org/licenses/>.
PDP10 ELF Relocation
Relocation entries reside in section headers with
- sh_type equal to SHT_REL or SHT_RELA
- sh_info equal to the section header index to which the relocations apply
- sh_link equal to the section header index of an associated symbol table
Section start addresses are associated with symbols of type STT_SECTION
and STB_LOCAL bindins. These symbols are used for relocation.
Symbols with st_shndx SHN_ABS have absolute values not affected by relocation.
Symbols with st_shndx SHN_COMMON record their alignment constraint as st_value.
Otherwise, in relocatable files, st_value is an offset into the section
identified by st_shndx.
Relocation entries (ELf36_Rel/Rela) consist of
- r_offset: offset into the section identified by the Shdr's sh_info at
which to apply the relocation
- r_info: records the index of a symbol table entry, in the symbol table
identified by the Shdr's sh_link, and a processor-specific relocation
type which is applied to the symbol's value
- an additional constant addend for the relocation:
* in SHT_RELA relocations stored in the r_addend field in Elf36_Rela
* in SHT_REL relocations stored in the location to be modified
A: the addend used to compute the value of the relocatable field, given
by r_addend (SHT_RELA) or the initial contents of the field (SHT_REL)
P: the place being relocated, given by r_offset
S: the value of the symbol referenced by r_info
B: the base address at which a shared object has been loaded into memory
during execution
G: the GOT offset of the relocation entry's symbol
GOT: the address of the GOT
L: the place of the PLT entry for the relocation entry's symbol
Relocation Types (8 bits)
Name Value Field Type Calculation
R_PDP10_NONE 0 none none
R_PDP10_IFIW 1 uint36 (S+A) >>u 2
This produces a local 18-bit address of a global 36-bit word, into the right half-word
of an instruction word or instruction format indirect word (IFIW).
The result has bits 0 to 17 preserved from the original contents of the place, P,
and bits 18 to 35 containing an 18-bit address.
Bit 13 (I) will be set by the linker or loader if the word is in a different section
than the place, P.
(S+A) must be word-aligned.
The place, P, must be word-aligned.
R_PDP10_EFIW 2 uint36 (S+A) >>u 2
This produces an extended format indirect word (EFIW) containing a global 30-bit word address.
The result has bit 0 set to zero, bits 1 (I) and 2 to 5 (X) preserved from the initial
contents of the place, P, and bits 6 to 35 containing the address (Y = (S+A) >>u 2).
(S+A) must be word-aligned.
The place, P, must be word-aligned.
R_PDP10_LOCAL_W 3 uint18 (S+A) >>u 2
This produces a local 18-bit address of a 36-bit word in the same section as the place, P.
(S+A) must be word-aligned.
The place, P, must be half-word aligned.
R_PDP10_LOCAL_B 4 uint36 (S+A) >>u 2
This produces a one-word local byte pointer to a 9-bit byte in the same section as the place, P.
Its P field is set to (3-((S+A)%4))*9, and its S field is set to 9.
Its bit 12 and I and X fields are set to zero.
The place, P, must be word-aligned.
R_PDP10_LOCAL_H 5 uint36 (S+A) >>u 2
This produces a one-word local byte pointer to an 18-bit halfword in the same section as the place, P.
Its P field is set to (2-((S+A)%4))*9, and its S field is set to 18.
Its bit 12 and I and X fields are set to zero.
(S+A) must be halfword-aligned.
The place, P, must be word-aligned.
R_PDP10_GLOBAL_B 6 uint36 (S+A) >>u 2
This produces a one-word global byte pointer to a 9-bit byte.
Its PS field is set to 070+((S+A)%4).
The place, P, must be word-aligned.
R_PDP10_GLOBAL_H 7 uint36 (S+A) >>u 2
This produces a one-word global byte pointer to an 18-bit halfword.
Its PS field is set to 075+(((S+A)%4)>>1).
(S+A) must be halfword-aligned.
The place, P, must be word-aligned.
[TODO: extend with GOT, PLT, and TLS relocations]
Assembler Syntax
symbol+offset
Produces no relocation if the symbol is absolute.
opcode accumulator,symbol+offset(index)
opcode accumulator,:ifiw(symbol+offset)(index)
Produces R_PDP10_IFIW.
.long symbol+offset
.long :w(symbol+offset)
Produces R_PDP10_LOCAL_W when assembling for -mno-extended,
and R_PDP10_EFIW when assembling for -mextended.
.long :b(symbol+offset)
Produces R_PDP10_LOCAL_B when assembling for -mno-extended,
and R_PDP10_GLOBAL_B when assembling for -mextended.
.long :h(symbol+offset)
Produces R_PDP10_LOCAL_H when assembling for -mno-extended,
and R_PDP10_GLOBAL_H when assembling for -mextended.
Names of relocation sections
.rel<name>
.rela<name>
SHF_ALLOC set if the relocated segment is loadable, otherwise off.
<name> is the name of the section for which the relocations apply,
e.g. ".rel.text" or ".rela.text".